EGSA Voltage Regulation and Excitation Questions and Answers — Questions and Answers
Question 1: When two generators are operating in parallel, what is the primary role of the Automatic Voltage Regulator's (AVR) droop setting?
- To control the sharing of real power (kW) between the generators.
- To ensure proportional sharing of reactive power (kVAR) between the generators. (Correct answer)
- To maintain a constant system frequency regardless of the total load.
- To increase the total voltage output of the paralleled system.
Correct answer: To ensure proportional sharing of reactive power (kVAR) between the generators.
In a parallel generator setup, the governor controls real power (kW) sharing, while the AVR controls reactive power (kVAR) sharing. The droop function on the AVR causes a slight reduction in voltage as the reactive load increases, which allows each generator to take on a proportional share of the kVAR load without 'fighting' the other units for voltage control.
Question 2: A generator set is equipped with a Permanent Magnet Generator (PMG) excitation system. What is the primary advantage of this system compared to a standard self-excited (shunt) system?
- It eliminates the need for an Automatic Voltage Regulator (AVR).
- It provides a lower initial manufacturing cost.
- It isolates the AVR's power source from the main generator output, improving performance under non-linear loads. (Correct answer)
- It generates DC voltage directly without the need for a rectifier.
Correct answer: It isolates the AVR's power source from the main generator output, improving performance under non-linear loads.
A PMG system uses a separate, small permanent magnet generator to provide a clean and isolated power source directly to the AVR. This prevents distortions and voltage dips from the main generator's output (caused by large or non-linear loads) from affecting the AVR's ability to control excitation, resulting in superior voltage stability and faster transient response.
Question 3: A technician is performing 'field flashing' on a self-excited generator that has lost its residual magnetism. What is the correct procedure?
- Applying AC voltage from the mains to the generator's output terminals while it is running.
- Connecting a drill to the output and spinning it in reverse to induce a current.
- Momentarily applying a DC voltage source, like a battery, to the exciter field windings while the generator is stopped. (Correct answer)
- Short-circuiting the stator windings to create a large current surge.
Correct answer: Momentarily applying a DC voltage source, like a battery, to the exciter field windings while the generator is stopped.
Field flashing is the process of restoring a small amount of magnetism to the generator's field poles. This is done by briefly connecting an external DC source, such as a 12V battery, to the exciter field windings to re-establish the magnetic field required to begin the voltage build-up process. It is crucial to follow manufacturer instructions to avoid damaging components.
Question 4: Which of the following conditions is a primary cause of generator overexcitation?
- A sudden, large increase in resistive load.
- Operating the generator at a frequency significantly higher than its rating.
- A sudden, complete loss of load (load rejection) while the AVR is in manual or fails to respond quickly. (Correct answer)
- A short circuit in the stator windings.
Correct answer: A sudden, complete loss of load (load rejection) while the AVR is in manual or fails to respond quickly.
Overexcitation occurs when the ratio of voltage to frequency (V/Hz) exceeds the generator's design limits, causing excessive magnetic flux. A sudden loss of load causes the terminal voltage to rise rapidly. If the AVR does not reduce the excitation current fast enough, this high voltage at a constant frequency leads to an overexcitation condition, which can overheat and damage the generator's core.
Question 5: A data center requires extremely precise voltage regulation (±0.5%) and fast response to load changes. Which type of Automatic Voltage Regulator (AVR) technology is best suited for this critical application?
- Analog AVR with droop compensation.
- Digital AVR. (Correct answer)
- Transformer-based voltage regulator.
- Electromechanical (carbon pile) regulator.
Correct answer: Digital AVR.
Digital AVRs use microprocessors to monitor and control the generator's voltage. This allows for much higher accuracy (often ±0.25% to ±0.5%), faster response times, and enhanced features like diagnostics and communication compared to older analog technologies. These characteristics are essential for protecting sensitive electronic equipment in applications like data centers.
Question 6: While commissioning two paralleled generators, a technician observes a large circulating current between them, and one unit shows a leading power factor while the other shows a lagging power factor, even with no external load. This indicates an issue with:
- the governor speed settings.
- the phase sequence of one generator being reversed.
- the AVR voltage setpoints or droop settings being mismatched. (Correct answer)
- the fuel supply to one of the engines.
Correct answer: the AVR voltage setpoints or droop settings being mismatched.
When paralleled, generators should have identical voltage setpoints and droop characteristics. If one AVR is set to a slightly higher voltage than the other, it will try to export reactive power (kVAR) to the system, causing it to operate at a lagging power factor. The other generator will absorb this kVAR, causing it to operate at a leading power factor. This flow of reactive power between units is known as circulating current.
EGSA Voltage Regulation and Excitation Questions and Answers
When two generators are operating in parallel, what is the primary role of the Automatic Voltage Regulator's (AVR) droop setting?